The invention concerns shaped bodies having pores, such as fibers, membranes in the form of hollow filaments, flat foils, tube foils and the like, a process for the production thereof, as well as an apparatus for performing the production process.
Membranes which can be employed for separation processes in various areas of use, for example, in the fields of medicine, pharmacy, foods chemistry, and also in technical areas, as well as processes for their production, have already been known for many years. The membranes are prepared from polymers such as regenerated cellulose, cellulose derivatives, and even from synthetic polymers such as polypropylene, polyesters, polyamides, polyurethanes, polycarbonates, halogenated polyolefins and the like. Depending upon the production conditions, the employed polymer and its characteristics, the membranes are suitable for osmosis and reverse osmosis, dialysis, ultrafiltration, microfiltration or for various other purposes. The factors that are critical for the suitability of a membrane with regard to the particular areas of use include its perviousness or permeability, its retention limit, as well as the selectivity. Moreover, characteristics such as mechanical strength, durability with regard to the medium of treatment, and hydrophilicity or hydrophobicity with regard to the treatment medium, among others, obviously play a role in this respect. It is additionally important that a membrane be capable of being left in use for as long as possible a time period during a separation operation without its becoming obstructed or having its characteristics alter during its employment to such an extent that the permeability and the selectivity no longer remain the same.
Of greatest significance insofar as the production of the membranes is concerned is that one be able to controllably regulate determined characteristics such as perviousness and selectivity, and to succeed in reproducibly obtaining these constant characteristics. Particularly in the field of medicine, nothing is namely worse than when the characterisitics of membranes fluctuate from one lot to another.
For many areas of employment, in particular for the treatment of water-containing mixtures not only in medical and pharmaceutical, but also in technical areas, it is advantageous to employ membranes which possess a certain hydrophilicity, such as e.g. membranes based upon polyamides. In other cases, it is considered valuable for the membranes to be very chemically resistant. Thus, for example, polyvinylidenefluoride (PVDF) represents a polymer which is durable even when employed with strongly acid and strongly alkaline aqueous solutions and against oxidizing media. Membranes of PVDF should thus also be employable mainly for processes for which other polymers, on account of their chemical nature, are less suitable or even completely unsuitable. The above mentioned characteristics of membranes such as permeability and selectivity also depend upon the pore structure of the membranes.
Membranes which display pores are generally prepared by means of working up of a polymer solution, for example by spreading out the polymer solution on a smooth support into a film and then allowing the solvent to evaporate, or by treatment with a liquid which does not dissolve the polymer but does dissolve the solvent, manufacturing the membrane structure by means of coagulation. In order to obtain pore structures as uniform as possible, coagulation techniques are not, however, particularly suitable. Even with the methods of production for membranes wherein the solvent is evaporated, there occurs a formation of a certain asymmetry within the membrane. Frequently even a so-called skin forms, which impairs the perviousness of the membrane.
In more recent times, processes for the production of membranes have been developed with which neither a coagulation by means of wet precipitation occurs, nor is the solvent evaporated from the solution by means of heating. Thus, for example, in DE-OS No. 28 33 493 a process is described for the production of porous hollow fibers employable as membranes, with which a homogeneous, single-phase mixture of a meltable polymer and a liquid inert with respect to the polymer, whereby the polymer and the inert liquid form a binary system which in a liquid aggregate state displays a range of complete miscibility and a range with a miscibility gap, is extruded at a temperature above the separation temperature into a bath composed completely or predominantly of the inert liquid which is also provided in the extruded mixture, the bath possessing a temperature below the separation temperature. By means of cooling, the formed hollow fiber structure is solidified. In order to obtain free, void pores, the inert liquid is removed after the solidification, preferably by means of extraction. It is possible with this technique to obtain membranes with an extensively isotropic structure in their interiors, and which in other respects possess a surface that is very smooth despite a high degree of openings.
Whether or not one can manufacture good membranes in outstanding manner with this technique, there occur difficulties if it is desired to work up mixtures the viscosity of which is either low, based upon their small concentration, or which contain polymer that, based upon its low molecular weight, leads to mixtures with low viscosity.
It has so turned out that even with these techniques certain difficulties can still occur when porous shaped bodies are supposed to be prepared from polymer compositions which are sensitive to mechanical stress during their working up. This susceptibility in the presence of mechanical stress is present in particular measure with low viscous polymer mixtures.
Even according to the more favorable techniques, with which a mixture of a homogenous liquid polymer composition upon cooling down runs first into a liquid two-phase area and then solidifies still above room temperature, it is difficult to work up these mixtures into shaped bodies having pores if the viscosity of the mixture to be worked up lies, before its extrusion through a nozzle, below a determined range, namely below about 15 Pa.s. On the other hand, it is frequently desirable to work with relatively low viscous mixtures in order to obtain specific development of the pore system.
It has also been proven that irregularities can occur during the production of the membrane when one works with a bath which is located in a customary tank. All movements such as wave motion, as well as changes in the bath with regard to concentration, temperature and so forth, influence the membrane formation and lead to membranes with differing characteristics. It is also difficult to so rotate such a bath that constant temperature and composition are guaranteed at the entry place of the extruded mixture.
A series of disadvantages occur also upon working with a spinning tube, such as described in DE-OS No. 28 33 493. Thus, for example, strong shear forces act on the forming membranes. Moreover, the process parameters in the spinning tube under which operation can be well performed, are strongly limited. This process thus allows working only within a relatively narrow viscosity and temperature range of the cooling medium, thereby limiting control of the pore size and pore structure. The above mentioned strong shear forces act particularly to disadvantage with low viscous polymer/liquid mixtures.
The manufacture of pore-displaying shaped bodies in the form of fibers is likewise complicated, and many disadvantages which occur with processes for the production of membranes also occur in this case. It is thus also difficult to obtain controllably determined pore structures. Another problem which concerns the production of such fibers is that, particularly for areas of use such as controlled delivery of active substances, specific adsorption and the like, fibers with precisely adjustable pore structures are required.
Accordingly, there still exists a need for an improved process for the production of pore-displaying shaped bodies, in particular the production of membranes with good permeabilities and selectivities.